macosx: added CoreAnimation based transitions to display the Drop Zone, switch between optical media and input devices

replaced the sprefs panel's legacy animation with basic CA effects

if libvlc users disable both mouse and keyboard events, chances are they're
trying to regain control over them. The WS_DISABLED flag exactly fits this need.

The only thing is that it is an overall flag that doesn't differentiate mouse and keyboard.

This greatly reduces the latency when pausing a legacy audio output.
Unfortunately, the audio output will keep polling for buffers though.

This should fix the remaining crashes and deadlocks when changing
the audio device or the channel map.

aout_PacketInit() does not allocate anything yet, but it will have too.

By the way, the error paths of the (untouched by the commit)
DirectSound plugin seem a bit suspicious to me...

This provides aout_PacketPlay(), aout_PacketPause() and
aout_PacketFlush() helpers for legacy audio outputs. They conveniently
match the callback prototypes of pf_play, pf_pause and pf_flush
respectively.